The present invention broadly relates to a new and improved construction of a magnetic system for a dynamic loudspeaker, suitable for sound reproduction with reduced sound distortion, i.e. enhanced sound fidelity.
In its more specific aspects the present invention relates to a new and improved construction of a magnetic system for a dynamic loudspeaker, comprising a magnetic path with an air gap, in which there is arranged at least one movable conductor which is movable in a predetermined direction of movement and which is connected to a current source.
Dynamic loudspeakers include current conducting conductors, most always arranged within a voice coil, which submerges into an annular magnetic field. The voice coil may, for instance, be connected to an amplifier which delivers electrical signals, which are to be transduced into acoustical signals. These electrical signals provide a deflection of the voice coil within the magnetic field, respectively in the air gap, while a diaphragm can be actuated by means of the voice coil.
For practical purposes, the emission of low frequency sounds of sufficient magnitude is only possible by means of voice coil systems. The sound reproducing fidelity of dynamic loudspeakers, however, is limited due to various causes, such as non-linear restoring force within the suspension system of the diaphragm, non-linear restoring force caused by the air cushion within the loudspeaker box or enclosure, non-uniformity of the magnetic field within the air gap, self-resonance of the diaphragm and the loudspeaker box, eddy currents and hysteresis within the yoke of the drive system of the diaphragm, and so forth.
It is well known that the sound reproduction fidelity of dynamic loudspeakers can be improved by certain measures within the voice coil or the diaphragm and its suspension system. It is further known that the fidelity of dynamic loudspeakers can be improved by the use of large and strong magnetic systems.
The disadvantages of these known solutions involve either high technical complexity, high cost and complications, or, for simple constructions, result in large, heavy and expensive loudspeaker systems. In principle, however, with magnetic systems of such large dimensions, the force exerted upon the voice coil, which is driving the diaphragm, is not exactly proportional to the current flowing within the voice coil. The current within the voice coil, on the one hand, generates an additional excitation within the magnetic circuit, i.e. within the entire magnetic path, comprising a suitable energizer, the pole pieces and the air gap, and, on the other hand, a variation of the magnetic field within the air gap alone.
The additional excitation within the magnetic path is a function of armature feedback, as is present in electrical drive systems. This additional excitation within the magnetic path, of which &.he magnitude is a function of direction of current flow within the voice coil, causes an increase or reduction of the magnetic field strength within the entire path, including the air gap. This in effect means, that with a constant current, the force propelling the voice coil outwardly is smaller than the force propelling it inwardly. Acoustically, this effect causes second order sound distortions. Furthermore, the center point of the oscillations of the voice coil becomes dislocated towards the inside with respect to the magnet, the effect of which is generally known as mechanical rectification. As a result the maximum possible deflection amplitude is also reduced.
The deviation of magnetic field strength within the air gap alone manifests itself through a decrease of magnetic field strength across the depth of the air gap. This decrease changes with the direction or with the amplitude of the current flow within the voice coil. Japanese Pat. application No. 51-138431 describes an arrangement which has the purpose of eliminating the disadvantages of the aforementioned variations within the magnetic field in the air gap and to provide means for eliminating current distortions within the voice coil. It has been proposed therein to provide means, in parts of which the magnetic flux is partially saturated, and such means are arranged immediately adjoining the air gap. Thus, there can be avoided that the current within the voice coil becomes adversely affected by a changing magnetic field within the air gap.
The aforementioned problems are also already known from U.S. Pat. No. 4,295,011, granted Oct. 13, 1981. The undesirable effects of the auxiliary magnetic field, which is generated by the moving, current-carrying voice coil, are sought to be improved by the provision of constructive measures at the adjoining surfaces of the air gap located between the poles. It is the purpose of these measures to focus the magnetic field within the air gap. In this proposal, as well as in the aforementioned Japanese Pat. application No. 51-138431, the magnetic field undergoes a modification within the air gap, in order to exert a positive influence upon the voice coil. Thus, the magnetic field within the entire magnetic path is not affected. There is merely modified the magnetic field within the air gap alone.
It is further known from German Pat. No. 594,490, granted Mar. 17, 1934, and the aforementioned U.S. Pat. No. 4,295,011, that saturation can be maintained within the entire magnetic circuit associated with the air gap. According to this U.S. Pat. No. 4,295,011 this brings improvements within the motion of the voice coil. According to the aforementioned German Pat. No. 594,490, such a saturated magnetic path can have a positive influence, whenever this saturated magnetic path is operated in parallel with an unsaturated magnetic path. The positive effect consists in the elimination of hum in the loudspeaker, whenever pulsating dc-current is used for exciting a magnetic circuit. It is, however, again very difficult to construct magnetic circuits which are saturable over the entire length of the magnetic path. A saturated magnetic circuit means that the magnetic reluctance has the same value as the magnetic reluctance within the air surrounding the magnetic circuit. The magnetic field in that case might choose different paths rather than the one leading through the air gap. In this case, additional solutions must be found in order to avoid these disadvantages, which, in turn, causes an increase of costs.